The present invention relates to a control for use in electrical water boiling appliances and in particular for electric kettles and jugs.
Electric kettles require a control to allow a user to heat water to a predetermined temperature, usually to boiling, at which point the kettle automatically switches off. A wide variety of controls exist to carry out this function, most commonly employing a heat-sensitive element such as a bimetallic or more recently a shaped memory alloy element which acts indirectly on an electric contact to break the electrical supply to the heating element.
An example of such a type of control is that of British Patent GB 1470364 of Otter Controls Ltd in which the bimetallic element is disposed above an opening receiving steam from the kettle body. The bimetallic element acts on an over-centre lever which in turn acts to break a pair of contacts connected in series with the heating element.
Commonly, such switches are integrated with additional operative parts having other functions in the kettle, such as thermal limiters that protect the kettle against switching on without water and after some structural arrangement allowing cordless connection. The present invention concerns only “stand-alone” switches which have the function of automatically switching off the kettle in response to steam, so-called automatic steam switches of the mechanical-thermal type.
In designing such a switch for a modem kettle the designer has to consider a variety of requirements the most significant from a safety point of view being the separation of electrically live parts from moisture generated inside the kettle during normal usage and during abuse such as over-filling and descaling, and the separation of electrically live parts from water spilled over the kettle exterior for example during the opening of the lid or clumsy filling under a water outlet.
It is also desirable that the switch reacts promptly to steam generation, and that the switch can be rapidly re-set even shortly after switching off, and these requirements influence the arrangement of the sensing element.
The size of the switch is also often of importance, the more compact the switch, the greater the freedom of the designer to create an aesthetically pleasing design which is not dictated by the size or shape of the functional components. However, as the size of the switch is reduced it then becomes increasingly difficult to meet the safety and operational requirements mentioned above.
With the requirements in mind, a vast array of switch designs have been proposed in recent years with varying functional and commercial success.
An example of a highly compact arrangement is given in DE 3801583 where a steam sensing actuator of memory alloy in a steam chamber acts through a lever on an over-centre trip lever which breaks an electrical contact pair.
A further arrangement is disclosed in GB 2213646 of Strix embodied in their R40 switch where the switch defines a shallow steam enclosure housing a dished bimetallic actuator which acts through a pushrod on a trip lever which in turn breaks contacts in a separate enclosure defined laterally adjacent the steam enclosure. The steam enclosure is here provided with an upper vent for the egress of steam. Although facilitate the cooling of the sensing element to allow rapid re-setting, the vent constitutes a source of dangerously hot steam with a risk of injury to a user.
The present invention is directed to providing a switch which is of improved compactness and yet which provides an excellent thermal response whilst being safe and reliable.